A detailed study is presented on the optimal use of helical slow-wave structures (both wire and slotted) for plasma production at microwave frequencies. For this purpose a feed optimization study was undertaken in which different feed structures located within the helical coils were used for exciting the helices. It is seen that the efficacy of plasma production using the different feeds depends directly on the relative importance of the field component excited by each feed for the slow-wave mode of the helix-loaded waveguide. The experiments show that the performance of the resonant helical coils is uniformly superior to that of the nonresonant one. The plasma so produced was characterized with respect to the microwave power and magnetic field at the antenna region. A second set of experiments was also performed, where a second helical coil (in addition to the helical antenna being used for plasma production) was used. The plasma produced by the helical antenna now flows into this long helical transmission line which acts like an extended wave-showing structure. It is found that using the long helical transmission line gives some improvement in the plasma density, the bulk electron temperature, and their radial profiles